1. Field of the Invention
The present invention relates to controllers for film deposition processes and in particular to controllers that use a quartz crystal as the sensor element.
2. Prior Art
Oscillating quartz crystals have long been used as sensor elements in the control of film deposition onto a substrate (web). (Leybold Inficon Inc. of East Syracuse, N.Y., presently offers a line of crystals and control circuits for precisely that purpose). Typically these crystals are part of an electrical circuit that causes the crystal to oscillate at a particular frequency. The crystal is disposed in the film deposition chamber and coated by the same process that applies the film to the web. As the crystal becomes coated, its frequency of oscillation changes and the rate of frequency change is a measures of the rate at which film is being deposited onto the web. When the detected frequency indicates that the rate of deposition is too high or too low, the deposition rate is changed until the rate of change of crystal oscillation is again within a specified range.
One of the major drawbacks of quartz crystal detection controllers, as known in the art, is that they require that the crystal be replaced after relatively short deposition runs due to the fact that after the crystal is coated to a certain level, it stops working. Some systems extend the controller operation time by using two crystals, with one being masked (inactive) until the first is coated to the point where it stops working and then the second crystal is unmasked (activated). But even in these dual crystal systems, coating runs last only five to twenty minutes before the crystals must be replaced. While these controllers have been acceptable for film deposition processes that can be completed within short time limits, (such as plate coating) they are not useful for processes that last longer than twenty minutes and which cannot be stopped prior to completion to change crystals, such as continuous coating of rolled material.
Thus, it is an object of the present invention to provide improvements in film deposition controllers including extending the length of time that a single crystal can operate as an accurate sensor of film deposition rate to as much as eight hours and more.
It is another object of the present invention to permit a pair of crystal sensors to operate in the same deposition chamber without either interfering with the operation of the other.
A further object of the present invention is to provide methods for extending the time that a quartz crystal can operate as a sensor of material deposition rate in a film deposition process.